Nuclear or scintillation or gamma cameras are conventionally used to perform SPECT studies. A patient ingests or is injected with a radiopharmaceutical, such as Thallium or Technetium, which emits gamma radiation from a body organ which is the subject of a medical study. The gamma camera detects the radiation and generates data indicative of the position and energy of the radiation which is then mathematically processed through a procedure known as reconstruction tomography (performed by computer) to produce pictures or scintigrams (two and three dimensional) of the body organ which is the subject of the medical study.
A typical gamma camera has at least two detecting heads. Each head contains an array of photomultipliers (PMT) which are arranged behind a scintillation crystal, typically Nal, which in turn is positioned behind a lead collimator. Gamma radiation passing through the collimator strikes the crystal which emits bursts of light or scintillations received by the photomultipliers which in turn generate analog signals indicative of the intensity of the light. The PMT analog signals are grouped, digitized, corrected and processed as data indicative of position, x,y, and intensity, z.
Traditionally, PET scanners are different from gamma cameras. In PET, radio nuclides, typically fluorine-18, carbon-11, nitrogen-13 and oxygen-15 are incorporated into substances such as glucose or carbon dioxide to produce radiopharmaceuticals such as FDG (Fluoro-Deoxy-Glucose) which are ingested by the patient. As the radio nuclides decay, positrons are emitted and they collide, in a very short distance, with an electron and become annihilated and converted into two photons, or gamma rays, traveling linearly in opposite directions to one another with each ray having an energy of 511 Kev. PET scanners typically comprise, laterally spaced rings which encircle the patient. Each ring contains detectors extending thereabout. A typical detector within the ring is a BgO crystal in front of a photomultiplier tube. Each ring is thus able to discern an annihilation event occurring in a single plane. The analog PMT signals are analyzed by coincidence detection circuits to detect coincident or simultaneous signals generated by PMT's on opposite sides of the patient, i.e., opposed detectors on the ring. Specifically, when two opposed detectors detect simultaneous 511 Kev events, a line passing through both detectors establishes a line of response (LOR). By processing a number of LORs indicative of annihilation events an image is reconstructed of the organ using computed tomographic techniques.
Although the literature oftentimes refers to one machine for performing PET and SPECT studies, the radiation events are different resulting in commercially different mechanisms for performing the studies. The PET scanner ring, while one dimensional, forms a complete solid angle about the patient and captures all events in a 2-D slice which can be stacked in accordance with normal tomographic techniques to produce a 3-D image. The gamma camera, while having a 2-D detector head giving it a power factor of one over the scanner ring, can not collect all the positron annihilation events about the patient and in fact, experiences an inherently low count rate which is significantly less than that achieved in a scanner ring. If the gamma camera retains its lead septa collimator (necessary for SPECT studies) photons emitted from non-normal positions of the body can not be accounted for and the low count rate further decreases to the point where the gamma camera simply does not acquire sufficient counts to perform any study with any degree of resolution. For this reason, commercial scanner rings are not fitted with collimators. This particular problem is discussed at some length in U.S. Pat. No. 5,323,006 in which a gamma camera is used to perform PET studies during a mammogram study since the breast can be viewed as being essentially compressed to a two-dimensional object which can be fitted between the detector heads.
Another fundamental problem arises from the different energies of the gamma rays sensed in a SPECT study compared with the significantly higher energy of the PET gamma ray. Thallium doped sodium iodide crystals are conventionally used in gamma cameras for SPECT studies while bismuth germanate crystals are conventionally used in PET scanners. For any given crystal having a given density, crystal thickness is sized to the energy of the ray. BgO crystals are not sufficiently sensitive and are generally unacceptable for SPECT studies. Increasing the thickness of the Nal crystal for positron generated 511 Kev photons will increase the sensitivity of the crystal for PET studies but lead to degradation of the gamma camera when used for SPECT studies. See generally U.S. Pat. No. 4,675,526. A more subtle but significant problem occurs with respect to fluoresence. See U.S. Pat. No. 4,864,140. Using a Nal crystal for PET studies significantly increases the fluorescence problem. Of course this problem is present only if the camera must be used to perform both SPECT and PET studies.
Perhaps one of the most serious problems affecting the commercial feasibility of using a gamma camera to perform both SPECT and PET studies stems from the signal processing capabilities of the camera. As noted above, PET detectors must determine first if a coincident event has occurred and then must determine a line of response for that event and accumulate a sufficient number of LORs to construct an image. Gamma cameras determine an x,y position of an energy ray and attribute an energy level to that position to determine a corresponding pixel definition on a scintigram. While coincident window circuits or electronic collimation to determine the existence of a positron annihilation event are well known, to simply equip a gamma camera with additional hardware and a second digital processing scheme for PET studies will result in a camera significantly more expensive than what can be justified on a commercial basis. The problem is compounded when it is realized that the signal processing scheme must also account for and process angular coincident events because if only photons normal to the detector head are accounted for, the photon count will diminish to an unacceptable level. Thus to adopt into a gamma camera a conventional PET scanner scheme for detecting positron annihilation events in a ring scanner will not necessarily produce the data needed to develop signals resulting from scintillations recorded in the detector heads of a gamma camera.
There are numerous PET scanner coincident detector schemes such as illustrated in U.S. Pat. Nos. 4,395,635; 4,864,140; 5,241,181; and 5,532,489 which determine if two photons struck a detector within a very short time of one another to establish a positron annihilation event. The '181 patent illustrates a conventional arrangement which samples packets of data and processes them through a digital signal processor to determine and identify valid events. The arrangement is sound but the speed of the processing time to determine a scintillation event is not acceptable for a gamma camera modified for PET studies. The '140 and '489 patents use an analogue circuit to determine a coincident event. Generally a triggering signal is developed from a scintillation event and passed with a time delay to an analogue coincident detector circuit which determines within a given time frame whether or not an annihilation event has occurred which is then passed to the digital signal processor for evaluation. In this manner the analogue signal is used to determine whether an event has or has not occurred which is then processed by the slower digital signal processor so that a fast acting coincidence arrangement is produced. This arrangement is also fundamentally sound. As a general concept, the accuracy of the coincident detection circuit is a function of the sophistication and expense of the circuit components so that a "tight" or "fine" detection circuit is obviously more expensive than a "loose" or "coarse" detection circuit.
Gamma cameras use electronics, as a rule, which are not as sophisticated or as expensive as that employed in PET ring scanners. Variations attributed to the gamma camera electronics will typically result in delays which can span anywhere from 30 to 50 nanoseconds for any one camera and not all of that delay can or is attributed to jitter and signal noise. To the extent that the variations can be statistically accounted for through calibration of a gamma camera to produce correction tables, calibration becomes critical to the successful operation of the camera. U.S. Pat. No. 5,272,343 shows an example of a system currently used to calibrate a ring scanner in which a positron source is orbited around the ring to produce better data for calibrating the ring. Such arrangement is not practical or desirable for a gamma camera of the type under discussion.
In the SPECT art it is also known to simultaneously perform imaging utilizing two different radio pharmaceuticals having photons of different energy levels such as technetium and thallium. Within the literature, there is disclosed in U.S. Pat. No. 5,323,006 the concept of performing a breast cancer study using Fluorine-18 in a modified gamma camera followed by placing collimators back onto the camera to thereafter perform a SPECT study. The literature does not disclose performing simultaneously a cancer study using PET images and SPECT images of an organ sequentially taken in one session to generate different images within the same time frame although the medical community could well use such studies in treating the patient.